Cast metal articles are made at a foundry by introducing molten metal into a mold from a relatively large volume of molten metal (i.e., a melt). It is well known to adjust the chemical composition of a casting metal to improve the mechanical and physical properties of an article cast from the metal. This is accomplished by the addition of one or more metallurgical additives or modifiers.
Some additives or modifiers “modify” the way a cast material solidifies. The desired effects of the modifying material can be observed at the microscopic or metallographic level. The size, shape and crystalline composition of the micro-constituents of the solidified alloy can be influenced through the addition of small amounts of “modifiers.” These modifiers typically exhibit little or no “alloying” effect, in the sense that they have negligible impact on the chemical composition of the melt. In this regard, the melt is modified by liquefying and dispersing the modifiers in the melt, before it is used to fill a mold.
Molten aluminum can be alloyed by the addition of alloying elements. Common aluminum alloying elements include copper, iron, magnesium, manganese, nickel, silicon, tin, and zinc. These alloying elements are typically added to the molten melt to formulate a specific aluminum casting alloy. Modifying elements, alloys and compounds may also be dispersed in the aluminum melt. Such additives include antimony, beryllium, boron, calcium, phosphorus, silver, sodium, strontium, titanium, titanium boron, vanadium, zirconium, and other elements and compounds of elements.
Strontium, in particular, is known to have a notable impact on the silicon morphology, porosity distribution and porosity volume in a casting, positively influencing the mechanical properties and the functional aspects (e.g., leak prevention) of the cast article.
Metallurgical modifiers, including strontium, are conventionally added to an aluminum melt in relatively small bricks, pellets or tablets that are subsequently liquefied and dispersed in the melt. It is also known to include these additives by moving an alloy wire into a stream of the molten metal. The introduction of the additives to the melt is typically performed at the holding furnace, launder, or ladle and at a predetermined time prior to pouring the melt into the mold.
Control over the time that elapses between the introduction of the modifier to the melt and use of the melt is important because the desired and beneficial effects of metallurgical modifiers that are dissolved in a melt diminish with the passage of time. This phenomenon is known as fade. Such degradation of the efficacy of the metallurgical modifier is an undesired result and reduces the effective concentration of the metallurgical modifier in the aluminum melt. Consequently, to counter the negative impact of fade, it is often the practice to over-treat the molten melt with strontium or other modifiers. When dealing with the metallurgical modifier strontium, in particular, it is believed in the foundry industry that the modifying alloy of strontium must be added to the melt before the metal is introduced to the mold so that the strontium can disperse throughout the melt.
Foundry practices that improve both the manner in which metallurgical modifiers are introduced into the melt and their resultant effectiveness in enhancing the mechanical and physical properties of the cast article, therefore, continue to be sought.